Preheat oven to 350°F. Grease a metal 9- x 13-inch baking pan with cooking spray. Line the bottom with a fitted rectangle of parchment paper. Set aside.

In a bowl, stir together flour, kosher salt, baking soda, and baking powder. Set aside.

In the bowl of a stand mixer, cream together butter, brown sugar, and granulated sugar until lightened in color, about 3 minutes. Add egg, yolk, and vanilla. Beat on medium-high until fluffy and fully combined, about 4 minutes. Scrape down sides of bowl, add peanut butter, and mix for 1 minute to combine. Add flour mixture and incorporate on low until fully combined. Add oats and mix to combine.

Spread dough evenly in prepared baking pan. Bake until the edges have slightly browned and a toothpick inserted comes out clean, about 22 minutes. Remove from oven and immediately sprinkle surface with chopped chocolate. Allow to sit for 2 minutes before using a spatula or spoon to spread melted chocolate evenly across surface. Sprinkle chopped peanuts and flaky salt across melted chocolate. Refrigerate until chocolate has set and bars have cooled, about 30 minutes. Cut 4 long horizontal rows and 3 short vertical rows and use an offset spatula to remove the Peanut Butter Fingers from the pan.